1. Field of the Invention
The present invention relates to a sheet switch to be used for a thinned electronic instrument such as a mobile phone or the like, a sheet switch module which is configured to add an illumination function to the sheet switch and a panel switch in which the sheet switch module is installed.
2. Description of Related Art
A conventional key switch provided in an operational panel installed in each of various electronic instruments such as mobile phones, mobile information terminals, or the like, often includes a key top disposed to allow for pressing of each of a plurality of springs and an illumination structure to illuminate the key top. The illumination structure is configured such that each of the key tops is illuminated by a light source, for example, a light emitting diode (LED), or such that a group of key tops is illuminated by means of a light guiding plate by one or more LEDs, thereby allowing the position of each key top to be clearly recognized (for reference, see Japanese Patent Laid-Open No. 2004-69751, FIG. 9).
FIG. 19 illustrates one example of a conventional key switch 1. The key switch 1 includes a plurality of key switch portions 1a provided on a circuit board 2 and an illumination structure 1b to illuminate the key switch portions 1a. Each of the key switch portions 1a includes a central contact 3 disposed on the circuit board 2, a circumferential contact 8 disposed circumferentially of the central contact 3, a spring 4 disposed on the circumferential contact 8 to face the central contact 3, and a hey top 7 having a rod 7a which is disposed above and facing the spring 4. The rod 7a presses a top surface of the spring 4.
The illumination structure includes a light guiding plate 5 disposed above the circuit board 2 and a plurality of LEDs 6, each of which is disposed on the circuit board 2 to illuminate a side surface of the light guiding plate 5. The rod 7a of each key top 7 extends downward through the light guiding plate 5. In addition, wiring patterns (not shown) are formed on the circuit board 2.
In the key switch 1, the key tops 7 as a whole are lighted by the light guiding plate 5 illuminated by the LEDs 6.
Another known example of conventional key switch is a thinned key switch having an illumination structure developed in response to the recent trend towards thinning of electronic instruments (for reference, see Japanese Patent Laid-Open No. 2004-69751, FIG. 6).
FIG. 20 illustrates a sectioned structure of such a conventional thinned key switch 11.
The key switch 11 includes a plurality of sheet switch portions 19 provided on a circuit board 12 and an illumination structure to illuminate the key tops 17. Each of the sheet switch portions 19 includes a central contact 13 disposed on the circuit board 12, a circumferential contact 18 disposed circumferentially of the central contact 13, a spring 14 disposed to face the central contact 13, and a key top 17 disposed to face the spring 14.
The illumination structure includes a light guiding plate 15 disposed to cover the area above the sheet switch portions 19 and a plurality of LEDs 16, each of which is disposed on the circuit board 12 to illuminate one side surface of the light guiding plate 15.
In the key switch 11, the light guiding plate 15 is disposed between the key top 17 and the spring 14. The light guiding plate 15 as a whole is lighted by illuminating the side surface of the light guiding plate 15 with light emitted from the LEDs 16, thereby allowing a lower surface of each of the key tops 17 to be illuminated.
However, because each of the above-mentioned conventional key switches has the structure in which each of the key tops 17 which controls each of the springs is illuminated by the corresponding light guiding plate 15, there is a problem that the key top 17 and the light guiding plate 15 must be provided separately from the spring 14, and this results in a key switch 11 having an increased thickness.
In addition, in the conventional key switch 1, as shown in FIG. 19, because the rod 7a of each of the key tops 7 extends downward through the light guiding plate 5, the area of the light guiding plate illuminating the key tops is reduced, and a thickness of each light guiding plate 5 must be increased more than a certain value to allow sufficient illumination of the key tops. Increasing the overall thickness of the light guiding plate results in increased thickness of the key switch, thus making it difficult to achieve a thinned key switch.
On the other hand, in the conventional key switch 11 shown in FIG. 20, because the light guiding plate 15 is disposed between each of the key tops 17 and each of the springs 14, the light guiding plate 15 must be thinned and elastic in nature in order to allow for controlling the spring 14 by the key top 17; therefore it is not possible for the light guiding plate 15 to retain sufficient light therein. Consequently, there is a problem that the key tops 17 have low brightness and that variations in brightness are marked.
The key switch also has a structure in which a gap may arise between the light guiding plate 15 and each spring 14, and also between the light guiding plate 15 and each of the LEDs 16. If there is a gap in the circumference of the light guiding plate 15, light leaks trough the gap, leading to a problem of insufficient light illuminating each key top and insufficient brightness of the key tops.
Therefore, in the above-mentioned conventional key switches 1 and 11, it is not possible to obtain a thinned and effective key switch which illuminates the key tops 17 exclusively.